Some categories of water cooled and moderated nuclear fission reactor plants used for producing steam to generate electrical power, such as commercial boiling water reactors, utilize coolant water circulating pumps having drive motors located in casings outside of the reactor pressure vessel connected with pump impellers located within the reactor pressure vessel by means of drive shafts passing through the wall of the reactor pressure vessel. Thus, in nuclear fission reactor plants of this type, a number of internal impeller pumps driven by external electrical or other motors are positioned in the lower portion, or bottom of the reactor pressure vessel with each having a drive shaft extending through the wall of the coolant water containing reactor pressure vessel. This arrangement necessitates the requirement of a multiplicity of bore openings passing through the curved bottom portion of the reactor pressure vessel which contains a large volume, or head of circulating coolant water under high temperatures and pressure for submerging the fuel core and transferring heat away therefrom in producing steam for work.
The bore openings in the reactor pressure vessel for the entry of pump drive shafts are each formed with the vessel having a short, generally vertical tube-like section or hollow cylinder stub projecting vertically upward around the bore openings. In one design a hollow cylindrical housing supports the external drive motor casing and surrounds the drive shaft extending from the motor, and passing through the reactor pressure vessel wall with the drive shaft extending therefrom to the pump impeller for circulating coolant water with the pressure vessel. This hollow cylinder housing extends concentrically within the bore up into the pressure vessel tube section or hollow cylinder stub, and is thus secured such as by welding the upper annular end of the housing to the surrounding inside area of the bore opening tube section or hollow cylinder stub.
Due to the routine mechanical movement of such driven impeller pumps, including high speed rotation and inherent vibrations, which over a period of time can result in wear and metal stress and fatigue, components of this circulating system are periodically serviced and may be replaced to insure effective and safe operation of the nuclear reactor plants. Maintenance service of this nature may require replacement or repair welding of the hollow cylindrical housing unit which supports the drive motor housing and surrounds the pump drive shaft, and extends a distance concentrically into the pressure vessel bore opening tube section or stub.
Maintenance service of this circulating system, including either replacement or simply repair welding of portions such as the upper annular end of the hollow cylindrical housing unit and the encircling pressure vessel bore opening tube section or the weld between the two, must be carried out under the most cumbersome, and difficult circumstances. Namely, this maintenance undertaking need be performed within the lower portion of the reactor pressure vessel while retaining therein a substantial body of radioactive contaminated coolant water for covering and cooling the heat producing fissionable fuel core. Thus, the maintenance service, including repair welding of the upper annular end of the hollow cylindrical housing unit and the pressure vessel bore opening tube section or the weld between the two, must be carried out beneath a high head of water which is contaminated with radiation and wherein the reactor vessel coolant water cannot be permitted to escape through the pressure vessel bore opening tube section that provides entry up into the reactor pressure vessel and through which the coolant water circulating pump assembly is installed. Service operations under such stringent circumstances requires very complex procedures and special equipment and tools.